Jennifer Hauxwell

Commonly Rare and Rarely Common: Comparing Population Abundance of Invasive and Native Aquatic Species

Invasive species are leading drivers of environmental change. Their impacts are often linked to their population size, but surprisingly little is known about how frequently they achieve high abundances. A nearly universal pattern in ecology is that species are rare in most locations and abundant in a few, generating right-skewed abundance distributions. Here, we use abundance data from over 24,000 populations of 17 invasive and 104 native aquatic species to test whether invasive species differ from native counterparts in statistical patterns of abundance across multiple sites. Invasive species on average reached significantly higher densities than native species and exhibited significantly higher variance. However, invasive and native species did not differ in terms of coefficient of variation, skewness, or kurtosis. Abundance distributions of all species were highly right skewed (skewness>0), meaning both invasive and native species occurred at low densities in most locations where they were present. The average abundance of invasive and native species was 6% and 2%, respectively, of the maximum abundance observed within a taxonomic group. The biological significance of the differences between invasive and native species depends on species-specific relationships between abundance and impact. Recognition of cross-site heterogeneity in population densities brings a new dimension to invasive species management, and may help to refine optimal prevention, containment, control, and eradication strategies.

The relative role of environmental, spatial, and land-use patterns in explaining aquatic macrophyte community composition

Quantifying the relative role of environmental and spatial factors to understand patterns in community composition is a fundamental goal of community ecology. We applied a tested and repeatable point-intercept sampling method to aquatic macrophyte assemblages in 225 Wisconsin lakes to understand the ability of environmental, land-use, and spatial patterns to explain aquatic plant distribution and abundance. Using a variation partitioning framework in conjunction with Moran eigenvector maps we found that environmental, land-use, and spatial patterns explained 31% of total adjusted variation in aquatic macrophyte assemblages across the landscape. Environmental factors were the most important (contributing 34% of the total explained variation), but all sources of variation were statistically significant. Community composition varied from north to south along a gradient of alkalinity and from disturbed to undisturbed lakes, diverging according to whether disturbance was urban or agricultural. The large amount...

Testing a methodology for assessing plant communities in temperate inland lakes

Abstract We outline and test an aquatic plant sampling methodology designed to track changes in and make comparisons among lake plant communities over time. The method employs a systematic grid-based point-intercept sampling design with sampling resolution adjusted based on littoral area and lake shape. We applied this method in 72 Wisconsin lakes ranging from 6.5–245 ha in size, recording species presence–absence and depth at approximately 20,000 unique sample points. To assess how reductions in sampling effort might affect data quality, we used Monte Carlo simulations (100 iterations at each of 9 levels of sampling intensity) to reduce total lake sample points by 10% through 90% using a stratified random selection approach. Species accumulation curves were fit using the Michaelis-Menten 2-parameter formula for a hyperbola, and the predicted asymptote was similar to observed species richness. In a subset of lakes, oversampling (200% effort) did not yield significant increases in species richness. However, even a modest reduction (10–20%) in sampling effort affected species richness, while frequencies of occurrence of dominant species and estimations of percent littoral area and maximum depth of plant growth were less sensitive to sampling effort. In addition, we provide results of a power analysis for detecting changes in plant communities over time. Future applications of this protocol will provide information suitable for in-lake management and for assessing patterns in aquatic plant communities state-wide related to geographic region, hydrological characteristics, land use, invasive species and climate.

Whole-lake Herbicide Treatments for Eurasian Watermilfoil in Four Wisconsin Lakes: Effects on Vegetation and Water Clarity

Abstract Four pilot whole-lake herbicide treatments for extensive Eurasian watermilfoil (EWM) (Myriophyllum spicatum L.) infestations were conducted in Wisconsin between 1997 and 2001 using fluridone at a range of dosages (6–16 μg/L). Annual post-treatment data (4–7 years) were evaluated to assess (1) effects on exotic plants; (2) changes to native plant communities; and (3) effects on water clarity. Temporal shifts in treatment lakes were compared against natural fluctuations in untreated reference lakes. In conjunction with aggressive follow-up spot treatments with 2,4-D or manual removal, fluridone treatments provided between 1 and 4 years of substantial EWM relief, with the exotic ultimately re-establishing at pre-treatment levels or greater in 3 of the 4 lakes. Native plant communities shifted in all 4 lakes following fluridone treatment. The large decreases, outside the range seen in untreated lakes (first quartile of the reference lake distribution) for all treatment lakes containing EWM, Elodea canadensis, Ceratophyllum demersum, and Najas flexilis, strongly suggest a direct effect of the fluridone treatment. We observed large increases, outside the range seen in untreated lakes (fourth quartile of the reference distribution), for 1 of 2 treatment lakes with Potamogeton crispus, and 1 of 2 treatments with Chara spp. Secchi depth decreased significantly in 2 of the 3 lakes for which data were available. Future applications should consider, among other criteria, the dominant natives in the plant community, their sensitivity to fluridone, and potential impacts associated with decreased water clarity.

Evidence of negative impacts of ecological tourism on turtlegrass (Thalassia testudinum) beds in a marine protected area of the Mexican Caribbean

Many marine protected areas (MPAs) have been established in recent years. Some MPAs are open to tourists to foster environmental education and generate revenue for the MPA. This has been coined “ecological tourism”. Here, we examine the impact of ecological tourism on turtlegrass (Thalassia testudinum) health in one area of the “Costa Occidental de Isla Mujeres, Punta Cancún y Punta Nizuc” MPA in the Mexican Caribbean. A heavily visited location was compared with an unvisited location. Turtlegrass leaves at the visited location were sparser, shorter, grew more slowly, and had more epiphytes than at the unvisited location. Vertical and horizontal rhizomes of turtlegrass also grew more slowly at the visited than at the unvisited location. There is reasonable evidence to suggest that the observed differences are likely due to the deleterious impacts of novice and careless snorkelers. If continuing, these impacts could cause severe degradation of the visited areas in this MPA and, thus, changes in management policies seem in order.

Efficacy, selectivity, and herbicide concentrations following a whole-lake 2,4-D application targeting Eurasian watermilfoil in two adjacent northern Wisconsin lakes

Abstract The herbicide 2,4-D (2,4-dichlorophenoxy acetic acid) has been used to control the nonnative aquatic plant Eurasian watermilfoil (Myriophyllum spicatum; EWM) since the 1950s. Although published research evaluates the herbicides predicted and observed concentration and exposure times in both laboratory and field settings, few data are available evaluating selectivity and long-term efficacy as well as herbicide concentration behavior following large-scale, whole-lake applications. A controlled study was conducted on 2 adjacent oligo-mesotrophic northern Wisconsin lakes to determine the potential efficacy and selectivity of large-scale and low-dose 2,4-D applications. Initial 2,4-D concentrations in both treated lakes were approximately 100 μg/L higher than the nominal lakewide targets of 500 and 275 μg/L, respectively, and the herbicide dissipated and degraded more slowly than predicted. A lakewide regression model relating 2,4-D concentration at monitoring sites to days after treatment (DAT) found the mean half-life of 2,4-D to be 34–41 DAT, and the threshold for irrigation of plants not labeled for direct treatment with 2,4-D (<100 μg/L) was not met until 50–93 DAT. In the lake treated at the higher 2,4-D rate, EWM was not detected for 3 consecutive years posttreatment. Additionally, several native monocotyledon and dicotyledon species also showed sustained significant declines posttreatment. This study is the first to link field-collected 2,4-D concentration measurements to selectivity and long-term efficacy in EWM control following whole-lake management efforts. Although multiyear EWM control was achieved with these single low-dose applications, longer than expected herbicide persistence and impacts to native plants demonstrate the challenges facing aquatic plant managers and the need for additional field studies.

Relationships between water chemistry and herbicide efficacy of Eurasian watermilfoil management in Wisconsin lakes

ABSTRACT Frater P, Mikulyuk A, Barton M, Nault M, Wagner K, Hauxwell J, Kujawa E. 2017. Relationships between water chemistry and herbicide efficacy of Eurasian watermilfoil management in Wisconsin lakes. Lake Reserve Manage. 33:1–7. Eurasian watermilfoil (EWM) is an invasive aquatic plant with the potential to disrupt native aquatic plant communities, decrease property values, and affect recreational uses of lakes. Much time, effort, and money is put forth every year to combat the spread of this invader. We examined the long-term efficacy of EWM management using herbicide and derived an empirical equation to quantify long-term treatment success for the lakes in this study. We found that lakes with lower pH (<7.5), conductivity (<0.1 mS/cm), and total dissolved solids (TDS <0.1 g/L) exhibited more successful long-term management. Our results suggested that lake water chemistry may play a role in long-term herbicide management success of EWM.

Evaluation of large-scale low-concentration 2,4-D treatments for Eurasian and hybrid watermilfoil control across multiple Wisconsin lakes

ABSTRACT Nault ME, Barton M, Hauxwell J, Heath E, Hoyman T, Mikulyuk A, Netherland MD, Provost S, Skogerboe J, Van Egeren S. 2017. Evaluation of large-scale low-concentration 2,4-D treatments for Eurasian and hybrid watermilfoil control across multiple Wisconsin lakes. Lake Reserv Manage. 34:115—129. Herbicides have been utilized for decades for nonnative milfoil control; however, limited literature is available examining large-scale herbicide applications, especially for commonly used herbicides such as 2,4-D (2,4-dichlorophenoxy acetic acid). Twenty-three lakes were studied pretreatment and posttreatment to monitor large-scale and low-concentration (lakewide rate: 73—500 ν/L) 2,4-D dissipation and degradation patterns, and determine the efficacy and selectivity of these treatments for Eurasian watermilfoil (Myriophyllum spicatum; EWM) and hybrid watermilfoil (Myriophyllum spicatum × M. sibiricum; HWM) control. Measured mean surface concentrations averaged throughout the initial 2 weeks after treatment ranged from 119 to 544 ν/L. In addition, the threshold for irrigation of plants which are not labeled for direct treatment with 2,4-D (<100 ν/L by 21 d after treatment) was exceeded in 18 of the 28 treatments. Calculated 2,4-D half-lives ranged from 4 to 76 d, and herbicide degradation was generally observed to be slower in oligotrophic seepage lakes. Year of treatment reductions in milfoil frequency ranged from 4 to 100%, with sustained multi-year control observed in some lakes. While good year of treatment control was achieved in all lakes with pure EWM populations, significantly reduced control was observed in the majority of lakes with HWM populations. Several native monocotyledon and dicotyledon species also showed significant declines posttreatment, with variation in recovery observed over time. Although target species control was achieved with some of these treatments, variation in herbicide persistence, reduced control in many HWM populations, and nontarget impacts to certain native plants demonstrate the need for additional research and field studies.

A macrophyte bioassessment approach linking taxon-specific tolerance and abundance in north temperate lakes

Bioassessment methods are critically needed to evaluate and monitor lake ecological condition. Aquatic macrophytes are good candidate indicators, but few lake bioassessment methods developed in North America use them. The few macrophyte bioassessment methods that do exist suffer from problems related to subjectivity and discernibility along disturbance gradients. We developed and tested a bioassessment approach for 462 north temperate lakes. The approach links macrophyte abundance to lake ecological condition via estimates of taxon-specific abundance-weighted tolerance to anthropogenic disturbance. Using variables related to eutrophication, urban development and agriculture, we calculated abundance-weighted tolerance ranges for 59 macrophyte taxa and clustered them according to their tolerance to anthropogenic disturbance. We also created a composite index of anthropogenic disturbance using 20 variables related to population density, land cover and water chemistry. We used a statistical approach to set ecological condition thresholds based on the observed abundance of sensitive, moderately tolerant and tolerant taxa in each lake. The resulting lake condition categories were usually stable across multiple survey events and largely agreed with condition rankings assigned using expert judgment. We suggest using this macrophyte bioassessment method for federal water quality reports, restoration and management on north temperate lakes.